Reflection and absorption of Alfvénic power in the low‐altitude magnetosphere

Abstract

Results from a numerical study of the nonlinear interaction between large‐amplitude Alfvén waves and the low‐altitude auroral magnetosphere and ionosphere are presented. In the model the Alfvén wave encounters collisionless resistive dissipation when and where the effective parallel drift of electrons carrying the wave field‐aligned current density exceeds a critical threshold corresponding to onset of current‐driven microinstability. The resulting parallel electric field and parallel potential drop introduce frequency, amplitude, and perpendicular length‐scale dependence in the absorption of Alfvén wave power. Analysis of the power flow shows that (1) most of the power generated by a constant‐current source in the magnetosphere and carried by relatively small‐amplitude, high‐frequency, and short transverse wavelength Alfvén waves is reflected at low altitudes by the wave‐induced collisionless resistive layer (RL); (2) Alfvén wave power is absorbed primarily in the RL except when stimulated by a constant‐voltage generator, wherein ionospheric Joule heating dominates the absorption at large transverse length scales; (3) the absorption is high (≈0.8) at length scales of 10–20 km (ionospheric projection) for constant voltage source conditions; and (4) the relation between the wave‐induced parallel potential drop and wave field‐aligned current is linear with proportionality constant of order 109 Ω‐m2 over much of the range of interest. Correspondence between predicted properties and satellite observations is demonstrated.